An integrated pulp and fine paper mill combines, broadly, wood pulping, pulp washing, pulp bleaching, and paper making operations. In each of these operations, large quantities of water are necessary and, in each, the resulting water effluent contains pollutants necessitating treatment of the effluent prior to recycling for use or prior to returning the water into a watercourse.
Of all these operations, the major source of pollution is the outfall effluent waste stream from the bleachery. The bleachery outfall, in fact, of an integrated pulp and fine paper mill, as described above, constitutes about 40% of the total mill effluent. In more specific terms, this bleachery effluent, in the case of a chlorine-based bleachery, contains most of the chloride content rejected by the mill, the major portion of the total mill color outfall (greater than 60%), and a substantial amount of total mill Biological Oxidation Demand (B.O.D.). From this effluent comes a major portion of the primary sludge separated from the mill effluent.
Present procedures for treating such bleachery effluents to purify the same act to reduce some of the pollutants, such as those primarily contributing to the high B.O.D., but they are not capable of reducing the chloride content or the color. Both levels remain unsatisfactorily high. At the present time, there are Governmental environmental requirements calling for lowering the chloride content of an effluent stream prior to discharge to about 250 parts per million. In addition, meeting the environmental requirements of Federal, state, and local govenments, it has long been desired to recover the outfall constituents in the effluent as well as to be able to recycle the water for reuse in the paper-making operations in order to lower the large amount of water that are necessary for paper making and to reduce the energy required.